1. Field of the Invention
The present invention relates in general to an active matrix type display device which employs a display material such as liquid crystal and which self-contains a drive circuit, and more particularly to the technology of relaxing the bad influence which is exerted on the display by the noises generated in the drive circuit or the like to control the unnecessary radiated level within the various EMC (Electromagnetic Compatibility) regulations.
2. Description of the Related Art
In recent years, the technology of forming thin film transistors (hereinafter, referred to as “TFTs” for short, when applicable) on an inexpensive glass substrate has made rapid development. This reason is that the demand for active matrix type liquid crystal display devices has been increased.
The active matrix type liquid crystal display device is such that the TFTs are arranged so as to individually correspond in position to several millions of pixels which are arranged in matrix (hereinafter, referred to as “pixel TFTs” for short, when applicable), and the electric charges which are charged or discharged in or from each pair of pixel electrodes are controlled on the basis of the switching element function of the associated TFT.
In addition, the TFTs for driving the pixel TFTs (for convenience sake, referred to as “the circuit TFTs” for short, when applicable) are incorporated in the peripheral drive circuit, and a pixel portion for display having the pixel TFTs arranged thereon, and a drive circuit portion having the circuit TFTs arranged thereon are formed on the same substrate in order to promote the high integration.
In addition, the display device is desired to have the high display characteristics of further promoting the multi-gradation, the high resolution and the like.
In this connection, the noise which has become conventionally a problem is one of the factors which exert the bad influence on the characteristics to dominate the display characteristics and the electromagnetic radiation.
In order to solve the above-mentioned problem, heretofore, the measures have been adopted in which the circuit configuration for suppressing any of noises is designed.
In order to obtain the display having the high picture quality and the high definition, the required number of display pixels of the display has been increased year by year. For example, in the NTSC Standards, the number of pixels of about 400 thousand is required, and in the HDTV Standards, the number of pixels of about 2 million is required. In such a panel having a large number of pixels, the drive frequency for the video signal will be necessarily very high. Therefore, in order to drive such a panel, the clock signal which has a very rapid dot clock and which has the frequency (in the range of several tens Hz to several tens MHZ) several times as high as that of the video signal is inputted to the drive circuit. For example, the digital video signal (or the analog video signal) which has the frequency band of several tens to several MHZ and several kinds of clock signals which have the frequencies of several MHZ, several tens kHz and several tens Hz in correspondence to each of the circuits included in the drive circuit are inputted to a sampling circuit included in the drive circuit, thereby driving the liquid crystal panel.
In addition, in ideal, the signal rise time period (tr) and the signal fall time period (tf) are both zero or identical to each other (tr=tf). However, in actual, tr and tf are finely different from each other due to the various causes.
As described above, the noises which are generated by inputting at least one pair of signals in which the signal rise time period (tr) and the signal fall time period (tf) are finely different from each other to the drive circuit exert the bad influence on the display characteristics and the electromagnetic radiation, and hence the problem as will hereinbelow be described arises particularly in the case where the signal having the high frequency band is employed.
More specifically, in the drive circuit of the integrated liquid crystal display device which is formed on the same substrate as that of the pixel TFTs, a first clock signal, a second clock signal and a start pulse are all inputted to a shift register circuit and also a shift pulse is fed to the sampling circuit by a buffer circuit. In this connection, the first clock signal and the second clock signal have mutually the reversed phase relation established therebetween, and also each of the circuits is configured in such a way as not to produce the phase difference.
The signal rise time period (tr) and the signal fall time period (tf) of the actual signal are slightly different from each other. Therefore, when inputting the first clock signal and the second clock signal of high frequency which have no phase difference therebetween to the shift register circuit, those clock signals are not cancelled out each other so that the small noises are superimposed on each other to generate the noises each of which has the large amplitude and each of which has the sharp waveform as shown in FIG. 5. Those noises exert the bad influence on the video signal when displaying the image on the screen so that the level of the electromagnetic radiation is varied and hence it is difficult to control the level of the electromagnetic radiation within the EMC Standards. In addition, there is the possibility that the sharp noises each having the large amplitude exert the bad influence on other circuits as well.
Such phenomena occur in the drive circuit (such as a latch circuit, a memory circuit, or a counter circuit) to which the first clock signal and the second clock signal are inputted as well as the above-mentioned shift register circuit. In addition, this is not applied to the clock signal alone. That is, such phenomena occur in a semiconductor integrated circuit to which one pair of signals (which have the reversed phase relation established therebetween) are inputted in which the signal rise time period (tr) and the signal fall time period (tf) are finely different from each other.
As described above, there arises the problem that the noises each having a sharp waveform which have been generated in the shift register circuit or the like are superimposed on the video signal so that the voltage applied to pixel electrodes is varied to change the display.
As the means for solving that problem, heretofore, there has been adopted the configuration in which the circuit for making, with respect to one pair of signals, one signal aligned with the other signal to cancel out the noises is provided every circuit, or the configuration in which the ideal signal waveform is formed in which the signal rise time period (tr) and the signal fall time period (tf) become identical to each other.
However, the circuit configuration becomes complicated in which on the basis of the above-mentioned method, the ideal clock waveforms are formed, and the phase difference between one pair of signals is made just zero, and hence it is difficult to design the circuit. In particular, with respect to one pair of signals of high frequency, it is difficult to make one signal aligned with the other signal, and also it is very difficult to make one signal aligned with the other signal.